It is well known in various types of gas turbine engine, especially those in the aviation field, to employ internal cooling arrangements for aerofoil components such as turbine blades and guide vanes. The aerofoil component typically comprises a pressure wall and a suction wall, and has leading and trailing edges, with the walls defining at least one internal passage for supply of a cooling fluid, usually cooling air, to one or more internal cooling features in the form of one or more holes and/or slots for effecting film cooling as the cooling air passes therethrough and out thereof and onto the exterior of the component. The trailing edge portion in particular of such an aerofoil component is often difficult to provide with an efficient cooling arrangement, because of its narrow geometry and the limitations of conventional casting techniques in being able to reliably and accurately form the requisite one or more cooling holes and/or slots in such a region of the component.
One example of a known hole-based internal cooling arrangement for the trailing edge portion of a turbine blade is shown in U.S. Pat. No. 3,819,295. Here two sets of drilled holes provided in the trailing edge portion of the blade form passageways which link an internal cooling fluid (typically air) supply passage with the rear trailing edge of the blade. Each set of holes is angled with respect to the other such that passageways of one set intersect those of the other set, thereby forming a lattice with the intersecting nodes acting as turbulence promoters and area increasers for improved convective heat transfer from the blade body to the cooling fluid. However, this cooling arrangement is difficult, time-consuming and costly to manufacture. It also leaves large areas of uncooled material in the hub and tip regions of the blade where space is limited and thus holes cannot be drilled, nor even formed by casting owing to the too small a size they would need to have.
In contrast, slot cooling of various kinds for the trailing edge portion of an aerofoil component has been used in many known designs of turbine blades and guide vanes, and compared with simple multi-hole cooling arrangements, the use of a continuous internal slot feed between the internal cooling passage and the trailing edge at the rear of the component results in high performance film cooling with high cooling effectiveness. This is primarily a result of the slot feed producing a continuous cover of the cooling film, without gaps or spaces therein that typically arise from the use of rows of holes. Although in some existing arrangements based on the use of holes it may sometimes be possible to use double rows of holes which are staggered with respect to each other in order to enhance the cooling film cover, this strategy may be difficult to implement in practice at the trailing edge of an aerofoil component because of insufficiency of available space to accommodate such an arrangement.
One example of a known slot-based internal cooling arrangement for the trailing edge portion of a turbine blade is shown in U.S. Pat. No. 4,407,632. Here a trailing edge slot is formed with an internal array of pedestals extending across its width, wherein selected pairs of pedestals are connected by a barrier wall attached to either the pressure side or suction side of the slot. The barriers extend only part of the way across the slot in order to trip up, or interrupt, the thermal boundary layer of cooling air flow, thereby allowing improved heat transfer from the blade body to the cooling fluid. However, this design of cooling arrangement is characterised by many sharp edges to the various features within the slot, making casting thereof difficult and leading to reduced mechanical durability.
Another example of a slot-based internal cooling arrangement is shown in International Patent Application WO2005/083236A1. Here a blade comprises an inner space defined between two walls (suction side and pressure side), with a cooling fluid inlet at a leading edge and a cooling fluid outlet at a trailing edge so that the inner space forms a passage for cooling fluid to flow therethrough. The passage contains two sets of specially shaped and arranged ribs projecting inwardly from the respective suction and pressure side walls so as to form respective channels for cooling fluid to flow through the inner passage from the leading edge to the trailing edge. The respective channel flow directions are each at an inclined angle relative to the aerofoil radial direction and change in a smooth curve from the leading edge of the channels to the trailing edge of the channels. However, the channel directions in each set are at an inclined angle relative to each other in the proximity of the leading edge such that they intersect in this region, whereas in the region of the trailing edge the channels merge into each other to form common exit channels at the trailing edge. In between, the ribs in each set are connected at their respective intersections, but otherwise the flows in the channels can mix. However, this design of cooling arrangement is, like that of U.S. Pat. No. 4,407,632 above, again difficult to cast, owing to the complex arrangement of the ribs, and it is also not feasible to extend the arrangement specifically into the trailing edge region itself of the blade, where space is limited and casting cores present constraints of minimum sizes.
A further shortcoming of many known slot-based cooling arrangements, including those of U.S. Pat. No. 4,407,632 and WO2005/083236A1 discussed above, concerns the requirement to control the coolant mass flow through the slot if the cooling efficiency is to be optimised, which the above disclosed arrangements fail to do. This is because the pressure difference between the flow in the slot and the gas path external to the component needs to be above a predetermined minimum level in order to maintain the required coolant flow. However, current manufacturing techniques for aerofoil components in particular do not allow consistent enough production of trailing edge cooling slots which are thin enough to sufficiently control coolant mass flow on their own. For example, in the context of typical casting production methods, a very narrow slot would require a particularly narrow core, which would be fragile and easily fractured, making it commercially unviable for mass production.
There is therefore a need in the art for new and improved internal cooling arrangements in aerofoil and other components which utilise slot-based cooling, as well as methods for their efficient manufacture, which lead to improved control of coolant mass flow and thus pressure loss and resultant heat pickup during the passage of cooling fluid through such arrangements. It is therefore a primary object of the present invention to address this need.